1. Field of the Invention
The present invention relates to a method for pulling up a single crystal semiconductor such as silicon by the CZ method in which a diameter of the crystal to be pulled up is measured by CCD camera and the thus measured diameter is automatically controlled.
2. Description of the Related Art
When a single crystal is pulled up by the CZ method, there are provided two methods of controlling the diameter of the crystal body. One is an optical method, and the other is a weighting method. In the device for pulling up a single crystal, there is commonly provided a heat shielding plate at an upper position of the melt so that heat of radiation emitted by a heating means such as a graphite heater and also emitted by a surface of the melt can not be transmitted to the crystal body which is being pulled up.
When the crystal is pulled up by the CZ method, there is provided a straight body portion pulling-up process in which the crystal is grown to a columnar shape of a predetermined diameter, and also there is provided a process of forming the shoulder in which the diameter of the crystal is gradually extended from a small seed crystal to the diameter of the straight body portion described above.
According to the optical-type automatic diameter control system, the diameter of the crystal is automatically controlled as follows. A ring-shaped high luminance portion, which is generated on an interface between the crystal body and the melt, this ring-shaped high luminance portion will be referred to as a bright ring hereinafter, is photographed by a television camera. The thus obtained video signal is inputted into an image processing apparatus, so that the width of a portion of the bright ring can be measured. The thus measured value is used as a value representing the diameter of the crystal body, and automatic control is conducted so that the diameter of the crystal body can be made to be close to a target value.
A position at which the width of the bright ring is measured is conventionally determined at a position located before the largest width portion of the bright ring where the width is 60 to 80% of the largest width described before. This measuring position is conventionally fixed at a position by fitting the straight body portion as a reference position.
In the optical diameter control system, the following problems may be encountered. When the process of forming the shoulder and the process of forming the body are continuously controlled, the measurement can not be obtained because the bright ring can not reach the above measurement position.
Further, in the above measurement method, even when a position of the television camera is changed a little, the reproducibility of the position of the crystal and the position of measurement can not be kept, and the measurement value can not be precisely reproduced. For the above reasons, in the actual pulling operation, an operator measures the diameter with a cathetometer, and when an error of large magnitude is found, the operator corrects the target value, so that the diameter of crystal body is near the target value by manually operating.
There is provided a method of measuring the largest width portion of the bright ring in order to measure the diameter of the crystal body. According to this method, the measurement position automatically traces the largest width portion of the bright ring by the function of the image processing apparatus. Therefore, like the aforementioned method in which the measurement value reproducibility is not affected by the measurement position reproducibility, it becomes possible to measure the diameter with high accuracy. Further, even if the diameter is small, the largest width portion can be automatically traced so that the diameter can be measured.
However, in the case of a pulling apparatus having a heat shielding plate, when a bright ring of a growing crystal is photographed by a television camera, a position of the television camera is restricted by the heat shielding plate. Therefore, the television camera is set at a position by an angle of 70 to 80.degree. with respect to the melt surface. When the television camera is set at the position which forms such a large angle with respect to the melt surface, the following problems may be caused. When the crystal body is deformed a little or the diameter of the crystal body is varied in the process of pulling up the straight body portion of the crystal body, the largest width portion is shaded by the crystal body itself, so that the largest width portion can not be seen and also it is true that an actual diameter portion of a column could be seen only by the television camera being set on the axis of the column. Further, a ring-shaped image of high luminance is formed with a lower end portion on the heat shielding plate when light is reflected on the surface of the melt. This ring-shaped image of high luminance is located close to the bright ring. Therefore, it is impossible to discriminate between the ring-shaped image and the bright ring. For the above reasons, the measurement can not be conducted stably.